I looked a bit into meshtastic and was told that if a node was physically compromised then messages could be intercepted. That cant be right, right?
Why bother? Just jam the fucking hell out of it. Most critical infrastructure is not on the ISM bands.
There was a well known crypto weakness, CVE-2025-52464, that allowed man in the middle decryption of meshtastic traffic. It was fixed by a firmware patch that improved crypto discipline.
From what I understand no, the relay node has no access to the messages.
If you compromise sending or receiving node then sure, of course.